The present invention relates to a method of processing a selected surface in a semiconductor process chamber by contacting the selected surface with a reactant that reacts preferentially with the selected surface relative to another surface when the selected surface is at a different temperature from the other surface. The invention is particularly suited for use in rapid thermal processing (RTP) systems. More particularly, the invention relates to the use of nitrogen trifluoride (xe2x80x9cNF3xe2x80x9d) gas to clean selected surfaces in RTP process chambers, including in Rapid Thermal Chemical Vapor Deposition (RTCVD) process chambers.
During the course of semiconductor device fabrication, a wafer undergoes many steps of processing. Some steps involve heating the wafer hundreds of degrees Celsius above ambient temperature. Unnecessary heating may lead to defective devices being formed on the wafer. One reason for this is that heat causes substances, such as implanted ions, to migrate outside their intended location on the wafer. Migration and other things affected by heating are compounded by the length of heating time and by the number of heat steps. To avoid the problems associated with wafer heating, it is advantageous to use processes that minimize the time that a wafer is kept at an elevated temperature. This is the principle of conserving the thermal budget of the wafer.
In a conventional process chamber using a tube furnace, for example, it might take several minutes to raise the temperature of the wafer to a desired level. Because the walls and other surfaces in such process chambers are directly heated along with the wafer, such systems can be referred to as xe2x80x9chot wallxe2x80x9d systems. The heated walls and other surfaces in the chamber also increase the wafer""s cooling time. Thus, in hot wall process chambers a wafer may be subject to prolonged heating and cooling.
In contrast, RTP process chambers use a radiant energy source, such as high intensity lamps, to rapidly heat a wafer to a desired temperature in a matter of seconds. The energy source may raise the wafer""s surface temperature by 75-125xc2x0 C. per second. Other surfaces in the chamber are not generally heated by the radiant energyxe2x80x94for example quartz liners and windows in the chamber may be transmissive of the radiant energy frequencies or the energy may not be directed onto the surfaces. Because the radiant energy source does not directly heat these surfaces, RTP systems may be referred to as xe2x80x9ccold wallxe2x80x9d systems.
Because the rapid heating and cooling in RTP systems conserve a wafer""s thermal budget, it was expected that RTP systems would be widely adopted for every heat-based process. However, RTP systems have certain disadvantages that have not been adequately addressed. One significant disadvantage is that radiant energy may not be absorbed uniformly by a wafer. This results in temperature variations in and across the wafer. Temperature variations lead to non-uniform process results. RTP systems therefore require sensitive systems for monitoring and controlling the temperature of the wafer. One useful system uses a parallel plate with a reflective coating that reflects energy to the backside of the wafer. An optic fiber collects and transmits the wafer""s backside emissivity to a detector that translates the frequencies of energy to a temperature reading. Unfortunately, a significant disadvantage of this temperature monitoring system is its vulnerability to certain cleaning processes carried out in an RTP process chamber.
This problem and certain others addressed by the present invention are illustrated in the following example: the deposition of polysilicon on a wafer in an RTCVD process chamber. Deposition of polysilicon is a known step in the fabrication of certain semiconductor devices. One notable process using polysilicon is the formation of hemispherical grained silicon (xe2x80x9cHSGxe2x80x9d). HSG formations enhance the storage capacitance in storage devices such as
Dynamic Random Access Memory Arrays (xe2x80x9cDRAMSxe2x80x9d). Poor, irregular HSG formations result in unclean RTCVD process chambers. (Methods for forming HSG are described, for example, in U.S. Pat. Nos. 5,634,974 and 5,759,262 which are hereby incorporated by reference as if set forth in their entirety.)
An undesired side effect from polysilicon deposition is that the polysilicon is deposited not only on the wafer but also on other surfaces in the process chamber. For example, in RTCVD process chambers, it accumulates on, among other things, the top quartz window and quartz liner of the chamber. A build-up of polysilicon impedes the transmission of radiant energy through the window onto the wafer. It may also lead to high particle counts in the chamber, lowering production yields. Therefore, periodic cleaning of the process chamber is necessary.
NF3 is a powerful etchant, effectively etching polysilicon and certain other substances used in semiconductor processing. Use of NF3 for in-situ cleaning of a non-RTP semiconductor process chambers is described in U.S. Pat. No. 5,797,195, to Hullinget al, entitled xe2x80x9cNitrogen Trifluoride Thermal Cleaning Apparatus and Process.xe2x80x9d The ""195 patent describes a xe2x80x9chot wallxe2x80x9d system for cleaning semiconductor fabrication equipment, including quartzware parts. NF3 gas is heated from approximately 100xc2x0 C. to 650xc2x0 C. by the existing heat source for the process chamber. At the same time, the heat source also heats the other surfaces in the process chamber, which surfaces the NF3 gas is free to contact. The ""195 patent does not teach or suggest that the described cleaning system is suitable for use in xe2x80x9ccold wallxe2x80x9d, RTP systems, including RTCVD systems.
It would be desirable to use NF3 gas to clean RTP process chambers, particularly those used to deposit polysilicon in HSG production. However, NF3""s high reactivity has so far limited its usefulness in this regard. NF3 cleaning, until the present invention, has not proved suitable for use in cold wall systems, such as RTCVD systems, because certain components in RTCVD chambers may be damaged by the high activity of NF3 at higher temperatures.
As the temperature of NF3 increases, its reactivity increases, converting to reactive species in the form of ionic fluorine and/or free fluorine. The temperatures that occur in RTCVD processing can exceed 750xc2x0 C. At such temperatures, NF3 is so corrosive that it attacks stainless steel, quartz, and silicon surfaces, damaging critical components found in RTCVD process chambers. As mentioned, temperature-monitoring systems based on wafer emissivity are particularly vulnerable to damage. NF3""s action on such unintended targets may also create unacceptably high particle counts in the process chamber, lowering production yields.
If the NF3 cleaning is performed at lower temperatures, the etching rate is too low to effectively clean the system. This is because most of the hardware inside the process chamber. including the top quartz window, is heated by heat transmitted from the wafer, not by light used to heat the wafer. Therefore, if the temperature of the wafer is low. the top quartz window is even cooler. This results in unacceptably low etching rates.
For the foregoing reasons, a method is needed that allows a reactant introduced into an RTP process chamber to act preferentially on certain surfaces. Among other things, such an improved method would allow strong etchants, such as NF3, to clean polysilicon deposits from process chambers without damaging sensitive components in the chamber.
The present invention overcomes the disadvantages of the prior art by providing a method by which a reactant may favor a reaction with selected surfaces in a semiconductor process chamber. In so doing, the present invention overcomes the problem of using a reactant that would act desirably on some surfaces and detrimentally on other surfaces.
The present invention provides a method that will facilitate the cleaning of RTP process chambers, thereby better facilitating the adoption and use of RTP systems. More particularly, the present invention overcomes the inherent disadvantages of using a powerful etchant, such as NF3 gas to clean an RTP process chamber.
The present invention also overcomes the problems of high particle counts and lowered production yields that result if NF3 gas or other strong etchant is used to clean RTP process chambers. Accordingly, the present invention also improves the process of HSG formation in RTP process chambers.
In overcoming the aforementioned disadvantages in the art, the present invention improves the efficiency and yield of semiconductor device production. Particularly, it improves the efficiency and yield of producing devices in RTP process chambers. More particularly, it improves the efficiency and yield of producing HSG formations on wafers using RTCVD systems.
One embodiment of the invention is a method of processing surfaces in a semiconductor process chamber, comprising: selecting two different surfaces in the process chamber, each surface at a given temperature being capable of reacting with a reactant introduced into the chamber; creating a predetermined temperature differential between the selected surfaces by allowing a heated object in the chamber to transfer heat to one selected surface so that surface becomes the surface at the higher end of the temperature differential; contacting the selected surfaces with a reactant present in the chamber during the predetermined temperature differential between the selected surfaces; and allowing sufficient time for the reactant to react preferentially with one surface to a predetermined degree. The reactant may be removed from the chamber after the reactant has reacted with a surface to a predetermined degree.
A method according to the present invention may also include repeating the foregoing steps following the processing of a predetermined number of work objects in the chamber. The foregoing method also provides for the removal of deposits composed substantially of polysilicon from the surface at the higher end of the temperature differential.
In the embodiments of the present invention, a selected surface at the lower end of the temperature differential may be cooled by a cooling means. The cooling means may be a fluid cooling system in conductive communication with the surface at the lower end of the temperature differential. Components of a temperature measurement system in the process chamber may be cooled to protect them against action by an etchant or other reactant.
The present invention may provide for the process chamber to include a reflectivity plate comprising a selected surface. The reflectivity plate is kept at the lower end of the temperature differential during cleaning by an NF3 gas or other etchant. By keeping the reflectivity plate at a lower temperature, it is protected from being damaged by an etchant that prefers reacting with other surfaces in the chamber kept at a higher temperature.
The present invention may also provide that a radiant energy source transmits energy to a work object thereby heating it. The surface of the heated work object radiates heat to a selected surface that becomes heated to the higher end of a temperature differential. The invention also provides that polysilicon may be etched off a surface that is transmissive of the radiant energy used to heat a work object in an RTP process chamber. One such transmissive surface may be the window between the radiant energy source and a work object. In the present invention, the etching of the transmissive surfaces occurs after they are heated by another object in the process chamber. That other object may be heated by the radiant energy source. Quartz surfaces in the chamber, including quartz liners and windows may be etched accordingly.
In another embodiment of the present invention, a method of cleaning an RTP process chamber is provided that includes the steps of heating a selected absorbent surface in the process chamber with energy from a radiant energy source, the radiant energy passing through a transmissive surface between the radiant energy source and the selected surface; allowing a selected transmissive surface in the chamber to heat by energy transferred from the selected absorbent surface, after the absorbent surface is heated by the radiant energy source; and contacting the heated transmissive surface with an etchant while there is a predetermined temperature differential between the selected transmissive surface and another selected surface in the chamber; and allowing sufficient time for the etchant to react preferentially at the transmissive surface to a predetermined degree relative to the other selected surface. Preferably, the selected absorbent surface comprises a wafer. The etchant in this embodiment may be NF3 gas. The other surface may be on a component of a temperature measurement system, such as a reflectivity plate. This embodiment is suitable for etching deposits on the transmissive surface, including polysilicon deposits. This embodiment also provides that the other surface may be cooled by a cooling means so that it is at the lower end of the temperature differential. For certain processes, particularly cleaning polysilicon deposits with NF3 gas or a similar etchant, it is advantageous to heat the selected absorbent surface to at least about 650xc2x0 C. to about 750xc2x0 C. so that it transfers sufficient heat to the selected transmissive surfaces to establish an appropriate temperature differential. More particularly, NF3 gas may be used to clean polysilicon from the transmissive surface by providing a selected temperature differential between the surfaces of at least about 200xc2x0 C. to about 500xc2x0 C., with the temperature of the transmissive surface with the polysilicon deposits at the upper end of the differential and being at least about 650xc2x0 C.
In still another embodiment of the present invention, a method of in situ cleaning of a process chamber is directed to running production wafers through a process chamber for depositing silicon on the wafers; stopping production runs for cleaning the chamber when silicon has deposited on the liner or a window of the chamber to a predetermined degree; heating a selected absorbent surface in the process chamber with energy from a radiant energy source, the radiant energy passing through a transmissive surface between the radiant energy source and the selected surface; allowing a transmissive surface in the chamber to heat by energy transferred from the selected absorbent surface; and, after a selected temperature differential has been established between the transmissive surface and another surface in the chamber, contacting an etchant present in the chamber with deposits on the transmissive surface. In this embodiment, as well as other embodiments, the selected absorbent surface may be on a silicon-based wafer. And the etchant may be NF3 gas. In this embodiment, as well as others, the selected temperature differential between the selected surface and other surface may be at least about 200xc2x0 C. Preferably, the selected temperature differential between the surfaces is from about 200xc2x0 C. to 500xc2x0 C. The temperature of the surface with the polysilicon or other material to be etched is at the upper end of the differential. A useful upper-end temperature for etching the selected surface with NF3 gas is at least about 600xc2x0 C. A preferable upper-end temperature for etching with NF3 gas is about 600xc2x0 C. to about 750xc2x0 C. However, in other embodiments, the temperature of the surface at the upper end of the temperature differential may be in excess of about 1000xc2x0 C., depending on the materials to be reacted.
Another embodiment of the present invention contemplates cleaning a semiconductor process chamber using a gas etchant. This embodiment is directed to heating an absorbent surface in the process chamber heat by energy from a radiant energy source, the absorbent surface being heated from about 400xc2x0 C. to about 1500xc2x0 C.; allowing a selected surface in the chamber to heat by energy transferred from the heated absorbent surface; and cooling another surface in the chamber so that there is a temperature differential between the cooled surface and the heated selected surface such that an etchant in the chamber reacts preferentially with the heated. Here again, the etchant may comprise NF3 gas. The temperature differential between the surfaces is also preferably at least about 200xc2x0 C.
As in other embodiments, the etchant may be NF3 gas. Again, a suitable temperature differential between the selected surface and other surface is at least about 200xc2x0 C.
The embodiments of the present invention are suitable for use in a cold-wall process chamber, particularly an RTCVD process chamber, used to form HSG capacitors on production wafers. When cleaning a process chamber of polysilicon deposits the polysilicon/silicon dioxide etching selectivity ratio may be in the range of about 4/1 to about 7/1, which will allow cleaning without unacceptable etching of other components in the chamber.
Other embodiments will be apparent to persons skilled in the art, which embodiments do not depart from the spirit and scope of the present invention.